Mr Lavrov said the deadly firefight, which reportedly left at least three dead and others wounded, showed Kiev did not want to control "extremists".

He said: "The authorities are doing nothing, not even lifting a finger, to address the causes behind this deep internal crisis in Ukraine."

Video:Vladimir Putin Special Report

The self-declared mayor of Slavyansk has appealed to Russia's President Vladimir Putin to help residents by sending in troops.

This will fuel fears in the West, which accuses Moscow of stoking unrest, that the violence will be used as an excuse for Russia to seize more Ukrainian territory, in a repeat of the annexation of Crimea.

Russia also has tens of thousands of troops based along Ukraine's eastern border.

And concerns will only be heightened after Mr Putin signed a law making it easier for Russian speakers across the former Soviet Union to get citizenship.

US Vice President Joe Biden has arrived in Kiev for an official visit, where he is set to criticise Moscow.

Video:Gun Battle Threatens Easter Truce

The shootout, the cause of which is fiercely disputed, threatens to derail already shaky efforts to de-escalate the continuing crisis.

The deal reached between Russia, Ukraine, the US and European Union, demanded an immediate end to violence.

The Geneva accord called for illegal armed groups to disband, and for protesters to leave occupied government buildings.

But pro-Russian separatists, who have seized offices across eastern Ukraine, have shown no signs of moving on.

Western powers have warned Moscow of further sanctions if it fails to bring its influence to bear on pro-Russian activists.

Video:Easter Truce Broken In East Ukraine

Mr Lavrov pointed out the authorities in Kiev had failed to remove protesters camped out in the capital.

He said: "Buildings in Kiev that were violently seized at the beginning of the Ukrainian events have not been vacated. Streets have not been cleared.